Sky and Telescope - September 2018 - 37

BETTER ALIGNMENT

O

ne of the best-known methods for polar aligning
an equatorial telescope mount, the drift method,
dates back more than a century. At that time chemical photography was tedious, and only dreamers imagined
machines doing complex mathematical calculations. Now,
however, there's a new method for polar alignment that
makes use of digital cameras and computers.
It's based on an image taken with a camera attached to a
telescope mount and the polar axis turned while the shutter is open. The resulting image shows star trails that revolve
around the exact point in the sky where the mount's polar
axis is aimed. The user defines the rectangular coordinates
(pixel locations) at the end points of three selected stars and
enters them into a spreadsheet that calculates what, if any,
altitude and azimuth adjustments to the mount's polar axis
are necessary to align it with the celestial pole. These can be
converted to the necessary number of turns of the mount's
adjustment screws, thus allowing the user to approach the
pole very directly without much trial and error. The spreadsheet and detailed instructions for use are available free at
sternwarte-nms.de/ext-links/downloads.
The method is easy to use, quantitative, and exact. In the
calculation, an accuracy of 1 arcminute is attainable, corrected for the influence of atmospheric refraction. It is applicable both in the Northern and Southern Hemispheres, and
it is nearly foolproof since faulty star-position measurements
or incorrect star identifications produce an error warning.
A spreadsheet program such as Microsoft Excel or Open
Office is required, and an image-processing program is recommended to determine the pixel coordinates at the end-

points of the star trails. The
freeware program Fitswork
(fitswork.de) is very accurate for this purpose as it
calculates the center of the
point-spread function of the
star images. And the user can
transfer pixel coordinates to
the spreadsheet using copy
and paste.
It's also helpful to have a
planetarium program to aid in
identifying stars in your image
and obtaining their right
ascension and declination
values, though the spreadsheet
does provide data for a small
selection of stars.

by Jürgen Kahlhöfer

p To make the photograph used
for the alignment process, you
can attach the camera anywhere
on the telescope tube or moving
parts of the mount and point
it toward the celestial pole. A
sturdy camera mount is highly
recommended.

Photography and Evaluation
To make the necessary star-trail image, you need to attach a
digital camera with a standard or a short telephoto lens to
your equatorial mount or telescope tube and direct it at the
celestial pole. I recommend first taking a static image to help
with identifying stars - for example, a 15-second exposure at
f/4 and ISO 1600.
Next you make a time exposure while rotating the polar
axis. It's best to begin and end this image with a few seconds
of static exposure to clearly mark the star positions. In other
words, open the shutter, wait a few seconds before starting

HIP 5372

δ UMi

CA MER A M OUN T: DENNIS DI CICCO; G R A PHIC: JÜRG EN K A HLHÖFER (2)

Polaris

HIP 37391

p Left: As explained in the accompanying text, data for the spreadsheet is obtained by measuring star positions at the ends of trails made by
rotating the mount's polar axis while the camera shutter is open. Right: The author's spreadsheet calculates the amount of altitude and azimuth
adjustment of the mount's polar axis needed to align it with the celestial pole. Detailed instructions for using the spreadsheet are included
when the free spreadsheet is downloaded.

sk yandtelescope.com * S E P TE M B E R 2 018

37


http://www.fitswork.de http://www.sternwarte-nms.de/ext-links/downloads http://www.skyandtelescope.com

Table of Contents for the Digital Edition of Sky and Telescope - September 2018

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